Thin-film hard disc magnetic media are widely used in read/write memory devices in computers. Increasingly, there is an effort in the thin-film medium industry to achieve higher recording density (Howard). Among the magnetic properties which are important to a high recording density are:
(1) Coercivity, defined as the magnetic field required to reduce the remanence magnetic flux to 0, i.e., the field required to erase a stored bit of information. Higher coercivity in a medium allows adjacent recorded bits to be placed more closely together without mutual cancellation. Thus, higher coercivity is associated with higher information storage density.
(2) HF signal amplitude, which provides a measure of the signal amplitude recorded at a selected high frequency, typically, between about 4-8 MHz. Signal amplitude depends in part on the magnetic M.sub.r of the medium, the higher the M.sub.r, the greater the HF signal amplitude.
(3) Resolution, which provides a measure of the response of a read/write system. Resolution is typically expressed as a ratio (in percent) between the read voltages at HF and LF, where F is as above.
(4) Signal-to-noise ratio (SNR) which defines the ratio of signal amplitude, or peak-to-peak amplitude of a single pulse, as a function of recording frequency, to recording noise at that frequency. A HIGH SNR ratios, due to signal amplitude and/or low noise, are necessary for high density recording.
(5) The isolated readback pulsewidth, PW-50, measured at 50% of base to peak amplitude, indicates the degree of signal distortion at a given readback frequency due to overlap between adjacent signals. ideally, for high density recording, PW-50 should be reduced.
Thin-film media are typically prepared by sputtering an underlayer, such as a chromium underlayer, onto the substrate surface, then sputtering a cobalt-based magnetic thin film over the underlayer. A protective, lubricating carbon overcoat may be applied over the thin-film layer by sputtering.
The requirements for lower fly heights and higher recording densities have placed severe requirements on the substrate. Al/NiP substrates of thicknesses below 35 mils have shown significant problems in maintaining good fly characteristics below 4 microinches. As the thickness of the Al/NiP substrate is reduced, these substrates have also shown a greater susceptibility to handling damage.
Nonmetallic substrates, such as canasite.TM. (glass-ceramic) or glass substrates, have smoother surfaces and higher flexural strength. As a result, they are capable of providing superior fly properties and are potential replacements for Al/NiP substrates. Unfortunately, longitudinal magnetic media deposited on canasite.TM. substrates have generally exhibited lower outputs, higher noise and increased bit-shifts over comparable media deposited on Al/NiP substrates.